This work presents the behavior of solar radiation penetration in water of Tucuruí-PA reservoir before and after periods of its installation. The data were obtained in the years 1983 and 1986 near the barrage, during dry and wet stations. The Κ attenuation coefficient and the euphotic zone profundity were calculated for each series of data. Errors concerning to evaluation were analysed too. It was verifyed through analysis that the attenuation of solar radiation by water was bigger in rainy periods than in dry ones. This periodical behavior was in connection with the large quantity of suspensoids transported along year by the Araguaia-Tocantins system. There was a clear alteration in k values, with a decrease of the values. It was evidenced that sedimentation phenomenom began with diminution of water current velocity.